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Artificial Light

Plant and aquarium fluorescent lighting Balanced light refers to balanced amounts of red and blue light being available to a plant. The former is necessary for stem and root development, while the latter stimulates solid leaf growth, and thickening of the stem. Seedlings need light, up to 16 hours per day and, as a rough figure, some 20 watts per square foot.

For light-germinating seeds, start light only inches from the soil, and, as plants grow, gradually raise the light source, keeping it some two to three inches above the tallest seedling. Move them outside, or migrate them to the summer windowsill, as conditions warrant.

Ordinary incandescent bulbs produce red light predominantly, and inefficiently. Fluorescents are more efficient, and can provide better quality light. A minimum of two fluorescent lights, side by side, is required. Note that cathode decay causes tube performance to drop gradually over 6 months to a year, and that tubes should be replaced within this timespan, even though they still appear to perform well.

Cool white bulbs peak in the green part of the spectrum but are adequate for good growth; for more of a balance, use a warm white and a cool white bulb together. This provides somewhat more red light, and is more than adequate for almost all species.

Daylight tubes, or better, plant growth fluorescents, offer the best conditions for plant growth, at a higher price. The former produce a more natural white light, and are relatively inexpensive. The latter peak in the blue and red parts of the spectrum, producing a lovely purple glow. Wide spectrum plant growth tubes are often more pink than purple, and may be more acceptable esthetically. Sylvania, General Electric and Philips all make good tubes, and most should be available locally.

tomato seedlings under fluorescent lights

Finally, full spectrum tubes seek to emulate daylight as closely as possible, and are suitable for plant growth. Tubes with a colour temperature of about 5000K simulate equitorial light conditions; a colour temperature of 7500K most resembles the noon sun in temperate areas. Check with your local lighting supplier.

To start most seedlings, an adequate lighting setup consists of two side-by-side fluorescents, on a timer set to provide light for 16 hours per day, though a four-tube fixture is much better. Reflective foil should be used to enclose the seed-growing area, preventing significant amounts of light from being lost to the surrounding area, as well as helping to maintain an adequate humidity in the immediate area.

High Intensity Discharge lighting, one very specific form of artificial lighting, is covered separately.

Topics Referenced

High Intensity Discharge Lighting
On photosynthesis

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